Tests: Reduce usage of assertEquals for 6.9

[desrosj]

Follow-up to:

• #62278 (6.8)
• #61573 (6.7)
• #60706 (6.6)
• #59655 (6.5)
• #58956 (6.4)
• #57855 (6.3)
• #56800 (6.2)
• #55654 (6.1)
• #54726 (6.0)
• #53364 (5.9)
• #52482 (5.8)
• #38266 (5.7)

The assertEquals() test method does not check that the types of the expected and actual values match. This can hide subtle bugs especially when the values are falsey.

Tasks:

• Switch to using assertSame() when the type of the value in the assertion is important
• Replace overall usage of assertEquals() with type-strict assertion methods, with the aim of potentially removing its usage altogether

To help ease the effort of merging tests, changes should also be made upstream in the Gutenberg repository.

This PR updates the test test_wp_count_attachments_should_cache_the_result to check the properties of the object returned instead of checking the object with assertEquals. That way, we guarantee the expected properties of the object.

Track ticket: https://core.trac.wordpress.org/ticket/63169

This PR replaces assertEquals() with more appropriate, stricter assertions where possible and without making any changes to source.

For easier reviewing, commits have been separated based on the replacement assertion and any additional changes required to implement the stricter assertion.

Trac ticket: https://core.trac.wordpress.org/ticket/63169
Trac ticket: https://core.trac.wordpress.org/ticket/62278
Trac ticket: https://core.trac.wordpress.org/ticket/61573
Trac ticket: https://core.trac.wordpress.org/ticket/60706
Trac ticket: https://core.trac.wordpress.org/ticket/59655
Trac ticket: https://core.trac.wordpress.org/ticket/55654
Trac ticket: https://core.trac.wordpress.org/ticket/54726
Track ticket: https://core.trac.wordpress.org/ticket/63169